IR spectroscopy measurements may be useful for a variety of purposes including aerospace, automotive and industrial applications, as well as biological and bio-medical applications. For example, infrared (IR) radiation is readily absorbed by materials in association with relative motions (vibrations) of atoms such as carbon, hydrogen, oxygen and nitrogen. As such, IR spectroscopy measurements may indicate a condition of a wide variety of organic as well as inorganic materials.
For example, frequently it is necessary to determine the coating weight of a coating material on a substrate, to verify that the film coating weight is sufficient, including but not limited to, anodized coatings on metallic substrates. For example, aluminum aircraft parts often receive an anodized coating or a surface treatment including second metals for corrosion resistance and for enhanced paint adhesion, referred to herein respectively as an anodize coating and an Alodine coating. Generally, a uniform coating amount within an acceptable range is desired.
Prior art methods for measuring metal surface treatment film amounts (e.g., anodize and/or Alodine coating) include time consuming and destructive testing approaches that include coating the aluminum substrate, then weighing the substrate and coating, then stripping the coating from the substrate, and then weighing the substrate again. Because the test method is destructive, it cannot be used on the manufactured product. In addition, the currently known testing process only generates a spatially averaged coating amount for the sample. As such, the currently known testing process does not determine coating amount variations over an area.
Another approach includes the use of a portable IR device that includes an IR filtering device to filter IR signals from the substrate to monitor a narrow portion of the IR spectrum around 10.98 microns to determine a weight of the coating, as outlined in co-owned U.S. Pat. Nos. 6,784,431 and 6,984,825) which are hereby incorporated by reference. In this method, an infrared beam is reflected off the metallic substrate to form a reflected beam and the reflected beam is filtered to a predetermined wavelength band and then detected. The infrared energy of the reflected beam is compared with a predetermined value of infrared energy reflected off the metallic substrate without the anodize coating to determine an absorbance value for the anodize coating. The absorbance value for the anodize coating at a target wavelength of 10.89 microns is then correlated to an amount of the anodize coating.
Drawbacks in the above approach include the expense of the IR filtering device and relatively limited diagnostic information included in the IR spectrum employed including the narrow filtered portion of the spectrum detected and monitored at a target wavelength of 10.89 microns representing an Aluminum-Oxygen-Aluminum vibrational stretch. Differences in surface reflectivity between calibration standards and samples in question can lead to measurement errors when using filter IR methods.
Other infrared(IR) non-destructive methods in the prior art used to measure the properties of thin films include using IR absorbance to determine the amount of a chromated conversion coating on a metallic substrate (U.S. Pat. No. 6,794,631), determining an amount of chemical cure and amount of surface contamination (U.S. Pat. No. 6,906,327), determining the amount/thickness of an opaque coating on a substrate (U.S. Pat. No. 6,903,339) and (U.S. Pat. No. 7,223,977), and determining an amount of heat damage to a resin-fiber composite substrate (U.S. Pat. No. 7,115,869), all of which are fully incorporated herein by reference.
None of the above methods and associated devices, however, disclose a method or device that is suitable for performing IR spectroscopy including determining a coating weight and/or amount of a converted metal coating (conversion coating) such as an anodize and/or Alodine film coating on a metal substrate using robust multivariate calibration methods, particularly where a portable, real-time capability is desirable, such as in aircraft manufacturing, assembly, maintenance, and repair of aircraft.
Thus, there is a continuing need for improved IR non-destructive testing methods including a method or device that is suitable for performing IR spectroscopy including determining a coating weight and/or amount of a conversion coating such as an anodize and/or Alodine film coating on a metal substrate using multivariate calibration methods, particularly where a portable, real-time capability is desirable, such as in aircraft manufacturing, assembly, maintenance, and repair of aircraft.
Therefore it is an object of the invention to provide a method or device that is suitable for performing IR spectroscopy including determining a coating weight and/or amount of a conversion coating such as an anodize and/or Alodine film coating on a metal substrate using multivariate calibration methods, particularly where a portable, real-time capability is desirable, such as in aircraft manufacturing, assembly, maintenance, and repair of aircraft.